Laser speckle reduction and photo-thermal speckle spectroscopy

ABSTRACT

A method of reducing laser speckle by reflecting an infrared laser off a rotating diffuser and coupling the diffused light into a multi-mode optical fiber. Also disclosed is a photo-thermal speckle spectroscopy device having an infrared laser, a visible laser, a foam, and a camera. The infrared and visible lasers are focused on the foam, which causes the visible laser to scatter. A camera records the speckle pattern, which shifts when the IR laser is turned on. The related method of photo-thermal speckle spectroscopy is also disclosed.

PRIORITY CLAIM

The present application is a non-provisional application claiming thebenefit of U.S. Provisional Application No. 62/482,643 filed on Apr. 6,2017 by Robert Furstenberg et al., entitled “LASER SPECKLE REDUCTIONTECHNIQUES FOR MICROSCOPY AND SPECTROSCOPY,” the entire contents ofwhich is incorporated herein by reference.

CROSS REFERENCE

Cross reference is made to co-pending U.S. application Ser. No.XX/XXX,XXX, filed contemporaneously herewith, entitled LASER SPECKLEINSTRUMENT, by Robert Furstenberg et al., (Attorney Docket No.106032-US2) the disclosure and contents of which are incorporated hereinby reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to laser speckle reduction and to a methodfor photo-thermal speckle spectroscopy.

Description of the Prior Art

Due to their high brightness, infrared (IR) lasers (such as tunablequantum cascade lasers (QCLs)) are very attractive illumination sourcesin both stand-off spectroscopy and micro-spectroscopy. In fact, they arethe enabling device for trace-level spectroscopy. However, due to theirhigh coherence as laser beams, QCLs can cause speckle, especially whenilluminating a rough surface. This is highly detrimental to thesignal-to-noise ratio (SNR) of the collected spectra and can easilynegate the gains from using a high brightness source. In most cases,speckle reduction is performed at the expense of optical power.

When coherent light (such as light from a laser source) illuminates arough surface, the resulting image contains a granular pattern calledspeckle (Goodman, Speckle Phenomena in Optics, Roberts & Company,Englewood Colorado (2007)). A typical speckle pattern is shown inFIG. 1. The speckle grain size and distances from neighboring grains isoften very sensitive to changes in sample geometry, opticalconfiguration, and wavelength of light used. Speckle can be modeled byconsidering the speckle pattern to be due to interference from acollection of scattering centers. FIG. 2 illustrates this concept.

In many applications, speckle is a nuisance as it obfuscates the realsignal (e.g. reflectance, transmittance etc.) from the sample. Dependingon the speckle contrast (as defined by the fluctuations of speckleintensity), it can reach levels where the signal to noise ratio of thedetection system is speckle limited. Therefore, it is imperative toreduce speckle contrast but without an associated reduction insignal-to-noise due to lower laser power throughput. This is very hardto do as “de-speckling” invariably comes with a decrease in the opticalpower of the de-speckled laser light. Ideally, a speckle-reductionoptical setup will allow for tuning the amount of speckle reduction.

There are several strategies for reducing speckle. Spatial averaginginvolves combining multiple pixels to wash-out speckle contrast.Temporal averaging involves collecting signal over a longer time orcombining consecutive camera frames. Spectral averaging involvesreducing spectral resolution by smoothing, which reduces specklecontrast. Polarization averaging involves combining the signal fromilluminating with two polarization states of the laser. A de-specklingprocedure can involve a coherent illumination wavefront becoming anincoherent wavefront.

BRIEF SUMMARY OF THE INVENTION

The present invention provides for laser speckle reduction and forexploiting speckle from rough surfaces for the detection of traceamounts of chemicals. Laser speckle is very sensitive to small movementsin the imaging setup. Heating the speckle-inducing substrate causes itto expand and the resulting speckle to change. The amount of specklechange is proportional to the increase in temperature, which is in turnproportional to the infrared (IR) absorption spectrum of the analyte todetect. This provides an inexpensive and simple, yet highly sensitivedetection mechanism for trace amounts of analyte.

There are several advantages with the present invention. It is moresensitive than prior art techniques due to multi-pixel sensitivity. Ituses less expensive (visible) components. Also, it can be tailored tothe desired application.

These and other features and advantages of the invention, as well as theinvention itself, will become better understood by reference to thefollowing detailed description, appended claims, and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical speckle pattern.

FIG. 2 is an illustration of speckle pattern formation and its modeling.

FIG. 3 shows images from a thermal imaging camera of an infrared speckleoff a rough surface. Speckle exhibits the peculiar property that itappears focused in an otherwise defocused image.

FIG. 4 is a schematic drawing of a speckle-reducing setup.

FIGS. 5A-5C shows images of direct speckle observation by imaging theoutput of the multi-mode fiber onto the beam profiler. FIG. 5A shows animage with the diffuser with no spinning. FIG. 5B shows an image of theshiny side (without the diffuser) with spinning. FIG. 5C shows an imagewith a diffuser with spinning. A dramatic improvement in specklereduction is achieved by spinning the Infragold® diffuser. Also, it isdemonstrated that having a good diffusive reflectance properties of thespinning diffuser is essential (Infragold vs. its shinier, but stillsomewhat rough side).

FIGS. 6A and 6B show images after illuminating a rough surface withlight from the speckle-reduction unit. As shown in FIG. 6A, with thediffuser motor off, the speckle is still present. As shown in FIG. 6B,with the diffuser spinning, speckle is eliminated.

FIGS. 7A-7C compare the effect of different illumination modes onspeckle formation. As shown in FIG. 7A, using a free-space laser beamresults in a fully developed speckle. As shown in FIG. 7B, using afiber-coupled laser beam results in a dense speckle pattern with lowerspeckle contrast. As shown in FIG. 7C, using the speckle reductionapproach of a spinning diffuser prior to fiber coupling results in noevidence of speckle.

FIG. 8 shows an IR reflectance microscopy setup. For testing purposes, a127 μm thick platinum wire was used.

FIGS. 9A-9C show microscope images of a thin wire. FIG. 9A shows athermal image of a hot wire. FIG. 9B shows an active, laser reflectancemicrograph using a speckle reduction unit with the spinning diffuserstopped. FIG. 9C shows an active, laser reflectance micrograph using aspeckle reduction unit with the diffuser spinning (for maximum specklereduction potential).

FIGS. 10A and 10B show the influence of changing wavelength on thereflectance micrographs. As shown in FIG. 10A, when the illumination isless coherent, the micrographs are nearly identical. However, as shownin FIG. 10B, without good speckle reduction, the images change withwavelength considerably.

FIG. 11 shows a photo-thermal speckle spectroscopy setup.

DETAILED DESCRIPTION OF THE INVENTION

The prevent invention relates to speckle reduction approaches and theirability to reduce speckle contrast while at the same time preserving ahigh optical throughput. Multi-mode fibers, integrating spheres, andstationary and moving diffusers may be used for speckle reduction.Speckle-contrast can be measured directly by acquiring beam profiles ofthe illumination beam or, indirectly, by observing speckle formationfrom illuminating a rough surface (e.g. Infragold® coated surface) withan IR micro-bolometer camera. Speckle contrast reduction ischaracterized from spatial, temporal and wavelength averaging for bothCW and pulsed QCLs. Examples of effect of speckle-reduction onhyperspectral images in both standoff and microscopy configurations areprovided herein.

Experimental Setup

Speckle was generated by using a tunable QCL (“MIRcat” by DaylightSolutions) tuned to approximately 8μm. Direct speckle was observed byimaging of the output from an IR multi-mode optical fiber (“PIR400” byNewport; 400 μm core, 0.25 NA, 1 m, SMA terminated) by an optical beamprofiler (“Pyrocam III” by Ophir/Spiricon). Indirect speckle wasobserved by imaging the speckle pattern formed by reflecting the beamoff a rough (anodized aluminum part by 80/20 Inc.) by either a FLIR“E60” or FLIR “Photon Block 2” micro-bolometer. The micro-bolometerswere equipped with wide-angle lenses. Both micro-bolometers have abuilt-in 7-14 μm bandpass filter.

Speckle Identification

In this experiment, we observed the speckle pattern off a rough(anodized aluminum) surface using a FLIR “E60” thermal imaging camera,as shown in FIG. 3. To distinguish between speckle and some other causefor the observed pattern, the camera lens was defocused and it wasobserved that the speckle pattern remained in focus which is a knownproperty of speckle.

Speckle Reduction Setup

To reduce speckle formation, the laser beam needs to be conditioned. Oursolution used a two-step approach: First, we reflected the IR laser offa rotating Infragold® diffuse surface. The rotation insured that we gota decrease in speckle contrast due to temporal averaging. Next, thediffusely reflected light was collected by a lens and coupled into amulti-mode optical fiber for further speckle reduction. A schematic ofthe setup is shown in FIG. 4.

The output from the multimode fiber was imaged on the beam profiler andis shown in FIGS. 5A-5C. FIG. 5A shows an image with the diffuser notspinning. FIG. 5B shows an image of the shiny side (without thediffuser) with spinning. FIG. 5C shows an image with the diffuserspinning. It can be seen from FIG. 5A that the diffuser alone is notenough to de-cohere the beam to a satisfactory level. It is only afterthe diffuser starts spinning that we get a good speckle reduction, asseen in FIG. 5C. Without the diffuser (when we flip it to its shinyside), the spinning alone cannot remove speckle, as seen in FIG. 5B.Therefore, it is only the combination of diffuser and the temporalaveraging from its spinning that can reduce speckle to acceptablelevels.

To further test the de-speckling performance, the light was used toilluminate an anodized aluminum surface (80/20 Inc. part). Without thediffuser spinning, the speckle is clearly visible, as seen in FIG. 6A.With the diffuser spinning, speckle is eliminated, as shown in FIG. 6B.

In FIGS. 7A-7C, we compare side-by-side the effect of differentillumination modes (from completely coherent, free space illumination,to more incoherent illuminations) on speckle formation. In FIG. 7A, afully developed speckle is seen when using a free-space laser beam. InFIG. 7B, a dense speckle pattern and lower speckle contrast is seen whenusing a fiber-coupled laser beam. In FIG. 7C, the speckle is completelyeliminated, when using our speckle reduction approach of a spinningdiffuser prior to fiber coupling.

Speckle in IR Microscopy

In a microscopy configuration speckle reduction appears to be harder toachieve (Furstenberg et al., “Advances in photo-thermal infrared imagingmicrospectroscopy,” Proc. SPIE 8729, 87290H (2013); Furstenberg et al.,“Chemical Imaging using Infrared Photo-thermal Microspectroscopy”, Proc.SPIE 8374, 837411 (2012); and Furstenberg et al., “Photo-ThermalSpectroscopic Imaging of MEMS Structures with Sub-Micron SpatialResolution”, Mater. Res. Soc. Symp. Proc. 1415, 35-40 (2012)). What maybe an excellent speckle reduction in a stand-off spectroscopy becomesmarginally acceptable in microscopy. This may be due to the largenumerical aperture objectives and/or high magnification.

The schematic of the microscope used in this experiment is shown in FIG.8. The sample is illuminated with light from the de-speckling unit froma multi-mode fiber. This light covers a portion of the field view of thereflecting objective (bright-field illumination). The reflected light isviewed by a micro-bolometer array.

The performance of the microscope was tested by imaging a thin Pt wire(0.005″ (127 μm) diameter wire). Passive (thermal emission) differentialimages of the wire heated by an ac power supply are shown in FIG. 9A.FIGS. 9B and 9C show laser reflectance micrographs using illuminationfrom the speckle reduction unit with the spinning diffuser stopped (FIG.9B) and moving (FIG. 9C).

To better evaluate the speckle reduction performance, the wavelength ofthe IR laser was changed. Since the wavelength change is small, there isnot expected to be a significant difference in the observed micrographs.FIG. 10A shows a minimal change in the reflectance micrographs as thewavelength is tuned, indicative of good speckle reduction. On the otherhand, FIG. 10B shows that without proper speckle reduction of theillumination, the micrographs are wavelength dependent and produce animage that is not consistent with the object being imaged.

Exploiting Speckle

Instead of reducing speckle, sometimes you can take advantage of it.FIG. 11 shows a photo-thermal speckle spectroscopy (transmissiongeometry) set-up. An IR laser is toggled on/off, and a visible (red)laser is always on. The laser beams are focused on a foam, which causesthe red laser to scatter. A visible CMOS camera (without lens) recordsthe laser speckle pattern. One can see the speckle pattern shift whenthe IR laser is turned on. A hypothesis is that when the foam heats up,the cells expand; thus, a change in speckle pattern is observed on thecamera.

Photo-thermal Speckle Spectroscopy

The sample can be deposited (by solvent) into a foamy material (e.g.Vyon F polyethylene). The purpose of the foam is to have many scatteringcenters so it forms a good speckle pattern.

The sample can also be a liquid (inside the pores), or even a gas orvapor inside the pores (in this case, the foam would have to becontained inside a containment cell with IR and visible lighttransmitting windows for access of the probe (mostly UV, visible or nearIR) and pump (mid IR) beams.

The foam will have to be made of IR transparent material and be of suchthickness as to not absorb too much in the IR region. Thick foams willalways absorb a lot, even though the foam material is transparent.Therefore, thinner foams are preferred.

It may be feasible to reduce the absorption of IR light into the foam ifthe foam cells are elongated and oriented such that the polarizationstate of the laser will decrease coupling into the foam. The analyte ismore depolarized, so it will absorb at the same rate. Other ways tominimize absorption (through engineering the pores and/or shape andpositioning of the scattering centers to minimize interaction of IRlight with the substrate, but keep the analyte absorption at the samelevel are envisioned.

Another possible way of decreasing the foam IR absorption would be tofill the pores with a liquid that has similar index of refraction in theinfrared (but not at the wavelength of the probing visible laser) to thefoam. This way the, IR light can pass though the substrate, and the onlyway it would get absorbed is if the analyte itself (which is stuckinside the pores, or even dissolved in the liquid) absorbs it.

Using rough substrates (such as foam) is beneficial so we don't rely onthe analyte itself to provide a rough surface for forming speckle. Also,this way the speckle pattern will be similar, no matter the analyte andits morphology. Also, the amount change in speckle pattern will then beproportional to the analyte concentration.

Another possible substrate would be the thin polyethylene or Tefloncards used in FTIR spectroscopy. They have a milky color and diffusevisible light but are very transparent in the IR.

Any specially design substrate (made out of meta-materials or otherengineered surface) that suppresses IR absorption, but causes visiblelight to scatter (either in the reflected direction—surface scattering,or transmission—volume scattering).

Also, the substrate can also be a photo-thermal micro-bridge that flexesupon heating (like a bi-metal). The underside could be made rough toinduce speckle that can be imaged by using a visible laser on theunderside of the bridge.

If the sample consists of a number of particles on an otherwise smoothsurface, the speckle from the particles itself can be used. If substrateis also rough, the speckle will be a combination of the two. Given thedifferent roughness of the substrate and particle spacing, these twopatterns might be qualitatively different and therefore separable. Forexample, it is possible that the spatial frequency of the specklepattern from the particles is different from that of the substrate (use2D FFT to get spatial frequency map).

Both reflectance and transmission geometry can be used for a system.

Other ways of quantifying the difference in speckle pattern between 2images:

-   -   using the “curl” operator from vector calculus (or some other        means) to analyze the vortices in the speckle pattern.    -   2D fast Fourier transform (FFT) of the image will yield        information on the inter-grain distance (spatial frequency of        speckle grains).    -   temporal FFT of the image pixels (or group of pixels) to extract        amplitude at the IR laser modulation frequency.    -   if image is not uniform in illumination, adjust image (e.g.        rolling background subtraction with intensity correction) before        doing any image analysis.

Using shorter wavelengths (e.g. UV) will make the speckle pattern bemore sensitive.

Using UV or near IR (or longer wavelengths) of the probe laser wouldmake the probe beam invisible.

Other detection modalities: Use a CCD camera with a given integrationtime. Synchronize integration such that for half of the integration theIR laser is off and then it turns on for the other half If the IR lightis absorbed, and therefore the speckle pattern changes, we will record amixture of two speckle patterns. If the IR laser is not absorbed, thespeckle pattern in both halves of the integration time will be the same.It is expected that the combined speckle pattern in the former case willhave shorter average distance between speckle grain than the former.Other differences can be exploited as well. The advantage is that theCCD is only read out once and this will reduce readout noise.

The IR heating laser can be modulated while the speckle images arecollected. By applying a demodulation (at the frequency of the heating)of the processed speckle de-correlation data, we can more accurately(less noisy) determine the heating induced speckle fluctuations.

The previous method can be expanded such that multiple IR lasers can beused at the same time, but modulated at different frequencies. Thismultiplexing approach can speed up the collection.

Also, by changing the modulation frequency of the heating laser, we canget information of the thermal properties of the analyte (thermaldiffusivity, conductivity etc.).

Speckle formation can be enhanced by cycling the probe light back intothe substrate using beam splitters, cavities of similar.

Speckle interferometry: split the illuminating laser beam (using abeam-splitter) into two parts—one going through a substrate containingthe analyte, the other through a clean substrate only. Then, recombinethe two beams and observe the interfered beams with a camera. Process asdescribed herein. In another version, propagate the second beamfree-space (i.e. not through a clean substrate) and then recombine. Inthis case, to make the second beam the same size as the first, a beamexpander may be needed.

Tuning the IR laser in a continuous (e.g. linear) fashion within itstuning range and monitoring (by analyzing differences in specklepattern, or other means) the differential heating (between closelyspaced wavelengths) can recover the whole photo-thermal spectrum withoutthe need to measure a list of wavelengths. There will be a need tonormalize such a spectrum with the optical laser power delivered to thesample. In this modality, if a “fresh” background is needed for specklepattern comparison, the laser can be modulated (by chopper orelectronically, opto-acoustically or other means) to get an “on-off”illumination pattern.

All the modalities in this document (and all other documents referencedherein) could be done either in a microscopy or remote sensing(stand-off, proximal etc.) configuration as well. But the mainapplication tested was for a benchtop lab instrument for chemicaldetection.

Speckle can be imaged directly or by collecting the visible light with asuitable lens/objective. The goal is maximize speckle pattern change forthe given induce temperature change. Additional polarizer, filters orany other optics to achieve this increased speckle pattern change can beutilized.

While probe beam is usually visible. It is possible to envision a singlewavelength (probe=pump) experiment. The IR laser itself makes a specklepattern—this could be viewed using an IR camera.

On resonance, the light doesn't penetrate the particles (or film) ofanalyte. Off resonance it does. Presumably, this would cause a change inthe speckle pattern that can be distinguished from wavelength inducedchanges alone. The morphology of the speckle may be different. Theinter-speckle grain distance may change. In this case, simple specklepattern movements are not enough, as the wavelength would need to bechanged.

The above descriptions are those of the preferred embodiments of theinvention. Various modifications and variations are possible in light ofthe above teachings without departing from the spirit and broaderaspects of the invention. It is therefore to be understood that theclaimed invention may be practiced otherwise than as specificallydescribed. Any references to claim elements in the singular, forexample, using the articles “a,” “an,” “the,” or “said,” is not to beconstrued as limiting the element to the singular.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method of reducing speckle, comprising:focusing an infrared (IR) laser input beam onto a rotating diffuser; andcoupling diffused laser light into a multi-mode optical fiber.
 2. Themethod of claim 1, additionally comprising illuminating a sample withlight from the multi-mode optical fiber that covers a portion of a fieldview of a reflecting objective, wherein the reflected light is sent to amicro-bolometer.
 3. The method of claim 1, additionally comprisingilluminating a sample at a distance from the multi-mode optical fiberwith light from the multi-mode optical fiber.
 4. A photo-thermal specklespectroscopy device, comprising: an infrared (IR) laser that is toggledon and off; a visible laser that is always on; a substrate, wherein thesubstrate comprises an analyte to be tested; and a camera; wherein theIR laser and the visible laser are directed to the substrate causing thevisible laser to scatter, wherein the camera records a speckle pattern,and wherein the speckle pattern changes when the IR laser is turned on.5. The photo-thermal speckle spectroscopy device of claim 4, wherein thesubstrate is a foam.
 6. The photo-thermal speckle spectroscopy device ofclaim 4, wherein the substrate is a polyethylene foam.
 7. Thephoto-thermal speckle spectroscopy device of claim 4, wherein thesubstrate is a meta-material or other structure that is engineered tosuppress IR absorption.
 8. The photo-thermal speckle spectroscopy deviceof claim 4, wherein the change in speckle pattern between IRlaser-illuminated frames and non-illuminated frames is determined bycorrelating images recorded by the camera.
 9. The photo-thermal specklespectroscopy device of claim 4, wherein the IR laser is split such thata portion of it is directed to the substrate and the other partinterferes with the reflected speckle pattern before being recorded bythe camera.
 10. The photo-thermal speckle spectroscopy device of claim4, wherein photo-thermal speckle spectroscopy can be performed atdifferent wavelengths by discrete stepping or continuous tuning the IRlaser wavelength.
 11. The photo-thermal speckle spectroscopy device ofclaim 4, wherein the IR laser stays on and the IR laser wavelength iseither step-wise or continuously changed.
 12. A method of photo-thermalspeckle spectroscopy, comprising: directing an infrared (IR) laser thatis toggled on and off and a visible laser that is always on to asubstrate causing the visible laser to scatter, wherein the substratecomprises an analyte to be tested; and recording a speckle pattern witha camera, wherein the speckle pattern changes when the IR laser isturned on.
 13. The method of claim 12, wherein the substrate is a foam.14. The method of claim 12, wherein the substrate is a polyethylenefoam.
 15. The method of claim 12, wherein the substrate is ameta-material or other structure that is engineered to suppress IRabsorption.
 16. The method of claim 12, wherein the change in specklepattern between IR laser-illuminated frames and non-illuminated framesis determined by correlating images recorded by the camera.
 17. Themethod of claim 12, wherein the IR laser is split such that a portion ofit is directed to the substrate and the other part interferes with thereflected speckle pattern before being recorded by the camera.
 18. Themethod of claim 12, wherein the method is repeated at differentwavelengths of the IR laser either by discrete stepping or continuoustuning the IR laser wavelength.
 19. The method of claim 12, wherein theIR laser is stays on and the IR laser wavelength is either step-wise orcontinuously changed.